1. Field of the Invention
The present invention relates to low-NOx burners, and, in particular, to air-staged low-NOx burners.
2. Description of the Prior Art
Oxides of nitrogen (NOx) are produced from the burning of fuels during the normal operation of a typical burner. These oxides combine with hydrocarbons in the atmosphere, creating “smog”, which, when inhaled, may cause injury. Further, the U.S. Environmental Protection Agency, as well as state and local air pollution agencies, have passed certain environmental laws providing limitations and technological standards on the amount of NOx a facility may emit. These standards are continuing to become more and more stringent, creating a technological need for low-NOx burners.
Decreasing the NOx emissions from a burner is a well-known need. For example, U.S. Pat. No. 4,004,875 to Zink et al. (hereinafter “the Zink patent”) discloses a low-NOx burner concept that introduces secondary air to the hot face of the burner in addition to the primary air. In the Zink patent, primary air is provided in an amount that is insufficient to completely combust the fuel. The secondary air is introduced in a second stage to complete the combustion process. Overall, the use of staged air in this manner leads to reduced NOx emissions from the burner unit. Likewise, U.S. Pat. No. 4,347,052 to Reed et al. discloses the use of primary, secondary and tertiary air in predetermined stoichiometric proportions in order to stage combustion and, thus, reduce the production of NOx from the burner. Finally, U.S. Pat. No. 4,983,118 to Hovis et al. describes the use of air staging to reduce the production of NOx from a regenerative burner. The introduction of secondary or tertiary air in all of these burner concepts demonstrates the well-known usage of incomplete combustion to retard the production of NOx from the burner. This retardation occurs due to the overabundance of carbon dioxide, water vapor and methane in the burner mix at the initial stage.
As the environmental laws tighten, there is still considerable room in the art for technology that further reduces the production of NOx from industrial burners. While the above-referenced patents, among others, use incomplete combustion to reduce NOx, improvements over this design concept are in need.